TANE 26,1980 A NOTE ON A POPULATION OF AN UNDESCRIBED TEREBRATELLA (BRACHIOPODA: TEREBRATELLIDAE) FROM URUPUKAPUKA ISLAND, NORTHERN NEW ZEALAND.

by F.J. Brook

Department of Geology, University of Auckland, Private Bag, Auckland.

SUMMARY Terebratella n.sp. has a patchy distribution in the area studied, and is restricted to near vertical or overhanging rock faces. It occurs along with a dense epifauna of encrusting suspension feeders, and brachiopod valves commonly support an encrusting epifauna. Terebratella n.sp. is not a typical member of the 'rock face subcommunity' at Urupukapuka Island.

INTRODUCTION

During a visit by the Offshore Island Research Group (Auckland University Field Club) to the Bay of Islands in January 1980, the author had the chance to study a reasonably large living population of an undescribed species of the brachiopod genus Terebratella (Doherty 1976). In-situ observation of the brachiopods was limited to one SCUBA dive of short duration and thus much of the data in this paper resulted from analysis of collected specimens. The population of Terebratella n.sp. was collected between Hope Reef and the east coast of Urupukapuka Island (N12/750583, NZMS 1 Sheet N12 Bay of Islands, 2nd Edn. 1970) at a water depth of 10 m.

HABITAT

The brachiopods at the location studied are living in an area of broken relief. In-situ greywacke masses rise up to 8 m depth from a coarse shellsand and lithic gravel seafloor at 12 m depth. The area is exposed to oceanic swells from the NE, and water turbulence is strong during periods of onshore swells. Wave-induced turbulence is sufficient to move the seafloor sediment as evidenced by ridges and furrows. Flat upper rock surfaces are covered with red algae (Lithothamnion and unidentified red algae) and rare Ecklonia and sponges (Ancorina and (Polymastia). Shaded rock overhangs support a dense and varied encrusting fauna including sponges, hydroids, ahermatypic corals, tube-forming worms, bryozoa, ascidians and barnacles. This habitat parallels that of the rock face subcommunity of 145 Noble et al (1976). However at Urupukapuka Island brachiopods are found only on a few near vertical or overhanging rock faces. The population sampled from one overhanging rock face has a density of 100 - 200/m2. The patchy yet gregarious distribution of the brachiopods may be due to a short free-swimming larval period as shown for Terebratella inconspicua (Sowerby) by Percival (1944). Substrate material adhering to the pedicles of collected brachiopods indicates that most brachiopods were attached either directly to greywacke or epiphytically on other Terebratella n.sp. specimens (Table 1). Lee (1978) considers that two substrate types need to be considered with regard to brachiopod attachment. The macrosubstrate refers to "the type of sediment on which the brachiopod is found" (p 402, Lee 1978). The microsubstrate is the substrate to which the brachiopod pedicle is actually attached. Thus the macrosubstrate of Terebratella n.sp. at Urupukapuka Island is greywacke. Only 38% of the brachiopods collected were attached directly to the macrosubstrate. At least 50% of the specimens were attached to other encrusting organisms with the majority being attached to other Terebratella n.sp. individuals.

CLUSTERING

Although most brachiopods in the population are solitary, clusters involving up to 6 brachiopods occur (Table 2; clusters are formed when larvae of Terebratella n.sp. settle on and grow attached to members of their own species. Thus clusters consist of an older brachiopod with younger brachiopods attached to its valves). Doherty (1976) noted that Terebratella n.sp. does not form clusters as readily as the morphologically similar T. inconspicua (Sowerby). When clusters are formed, they do not include as many individuals as can be found in clusters of T. inconspicua. Clusters of the latter species can include more than 25 individuals (Doherty 1976).

EPIFAUNAL ENCRUSTATION

All mature specimens of Terebratella n.sp. from Urupukapuka Island have valves encrusted by epifaunal organisms. Epifaunal diversity generally increases with increasing shell size and mature specimens of Terebratella n.sp. are commonly completely encrusted. Fifty-one mature and sub-mature individuals have been assessed for presence/absence of the different epifaunal groups and these data have been converted to % frequency (Table 3). With the exception of red algae, the epifauna comprises solely suspension feeders. Hydroids and membraniporiform bryozoa are nearly ubiquitous as encrusting 146 Length of Pedicle Valve (mm) Fig. 1. Size frequency histogram of mature and submature Terebratella n.sp. from Urupukapuka Island (49 specimens). organisms and are among the first colonists of young brachiopod shells. The epifaunas of brachial and pedicle valves are very similar but there are differences in % frequency of some of the epifaunal groups (Table 3). Lithothamnion, an unidentified red alga, and Spirorbis occur on pedicle valves more commonly than on brachial valves. Conversely sponges and Hydroides occur more commonly on brachial valves. Most brachiopods in the population had the posterior-anterior axis perpendicular to the rock face. This would presumably allow them to produce ciliary feeding currents unimpeded by the epifauna on the surrounding rock face. However there was no observed preferred superposition of either brachial or pedicle valves. Thus the apparent valve preference shown by the above mentioned epifaunal groups cannot be related to a consistent growth attitude of Terebratella n.sp. with respect to the substrate.

Table 1. Relative abundance of microsubstrate types inhabited by Terebratella n.sp. at Urupukapuka Island (55 specimens). Microsubstrate type % Brachiopods attached

Terebratella 40 Greywacke 38 Unknown 12 Calcareous worm tube 4 Lithothamnion 2 Culicia (Coelenterata) 2 Membraniporiform Bryozoa 2

147 Table 2. Relative abundance of cluster sizes of Terebratella n.sp. from Urupukapuka Island.

Number of brachiopods in cluster % Relative occurrence 1 63 1 + 1 19 1+2 9 1+3 3 1 + 4 3 1+5 3

Le Barbera (1977) has shown that the Recent brachiopods Lacqueus californianus and Terebratulina unguicula can actively reorient their shells through an arc as great as 120° by rotating the shell about the pedicle. It is probable that Terebratella n.sp. is similarly able to reorient its shell. Such periodic alteration of orientation could produce the observed anomaly of both Spirorbis (photonegative larvae, Knight-J ones 1951) and phototrophic red algae occuring most frequently on pedicle valves. Ecological criteria influencing epifaunal settlement on the brachiopod valves probably include illumination, turbulence and the texture of the valve surface.

POPULATION STRUCTURE

Only larger i.e. submature and mature specimens of Terebratella n.sp. were collected from Urupukapuka I. (Fig. 1), owing to the difficulty of collecting juveniles. Size frequency distributions on T. inconspicua by Doherty (1976) show that populations are "normally skewed towards mature individuals". This is caused by the growth of

Table 3. Relative % frequency of epifaunal types encrusting Terebratella n.sp. at Urupukapuka Island (51 specimens).

% Pedicle % Brachial % Brachiopods Epifaunal Group valves valves encrusted encrusted encrusted Lithothamnion 56 44 26 Unidentified red alga 14 14 0 Sponges 50 20 36 Hydroid 06 80 86 Hydroides (Annelida) 14 6 12 Spirorbis (Annelida) 66 52 30 Membraniporiform Bryozoa 86 76 62 Celleporiform Bryozoa 8 8 6 Vinculariform Bryozoa 4 2 2 Terebratella 26 14 14 Hiatella (Bivalvia) 2 2 0 Barnacle 6 4 2 Amphipod 2 0 2 Ascidian 2 2 0 148 individuals to a certain size with subsequent increasing shell gibbosity (dorso-ventrally) and only minor increase in shell length. Thus mature individuals of differing ages can be of similar shell length. The population of Terebratella n.sp. is multimodal. This probably results from temporally-spaced spat falls producing distinct 'age classes' within the population.

CONCLUSION

At Urupukapuka I. Terebratella n.sp. occurs in association with a typical epifauna of shallow subtidal (5-20 m) shaded rock overhangs. The habitat parallels that in which subtidal T. inconspicua is commonly found. The population of Terebratella n.sp. has a lower density than subtidal populations of T. inconspicua (2000 - 3000/m2) described by Doherty (1976), and Terebratella n.sp. is absent from many apparently suitable habitats in the area studied. This cannot be attributed solely to a scarcity of hard surfaces for larval attachment. Both T. inconspicua and the Rhynconellid Notosaria nigricans (Sowerby) can have high density in habitats with dense epifaunal encrustation (pers. observ.). This sparse, patchy distribution of Terebratella n.sp. may relate to low efficiency of larval dispersal and colonisation.

ACKNOWLEDGEMENTS

I am grateful to Dr R.V. Grace for helping with fieldwork, and to Dr P.F. Ballance (University of Auckland) for reading the manuscript and suggesting improvements. I would also like to thank Robyn Thompson for typing the manuscript.

REFERENCES

Doherty, P.J. 1976: Aspects of the feeding ecology of the subtidal brachiopod Terebratella inconspicua. Unpublished MSc thesis, University of Auckland. 183p. Knight-Jones, E.W. 1951: Gregariousness and some other aspects of the settling behaviour of Spirorbis. Journal of the Marine Biological Association of the United Kingdom 30: 201-222. La Barbera, M. 1977: Brachiopod orientation to water movement 1. Theory, laboratory behaviour, and field orientations. Palaeobiology 3:270-287. Lee, Daphne E. 1978: Aspects of the ecology and paleoecology of the brachipod Notosaria nigricans (Sowerby). Journal of the Royal Society of New Zealand 8: 395-417. Noble, J.P.A., Logan, A. and Webb, G.R. 1976: The Recent Terebratulina community in the rocky subtidal zone of the Bay of Fundy, Canada. Lethaia 9:1-17. Percival, E. 1944: A contribution to the life-history of the brachiopod Terebratella inconspicua Sowerby. Transactions of the Royal Society of New Zealand 74: 1-23.

149